1. Field of the Invention
This invention relates to an apparatus and a method for forming weld joints having improved material properties and, more particularly, to a method of forming weld joints that induces residual compressive stress patterns to improve the material properties of the weld joint.
2. Description of Related Art
In the manufacture and construction of many types of structures, welding, such as gas welding, arc welding, resistance welding, thermite welding, laser welding, and electron-beam welding, has reduced or replaced the use of various types of fastening methods, such as bolting, riveting and the like. Such welding techniques either involve the complete fusion of material proximate to the weld joint thereby forming a liquid state which subsequently solidifies producing altered microstructures and properties, or the techniques involve a solid state welding process, which also produces a highly altered metallurgical state. The particular welding process best suited to joining two pieces of metal depends on the physical properties of the metals, the specific use to which they are applied, and the production facilities available.
Friction stir welding is a relatively new welding process for joining together workpieces of metallic materials and has been used in the aerospace, automotive, marine, construction, and other industries in the manufacture of equipment, structures, and the like. As illustrated in FIG. 1, friction stir welding involves inserting the threaded pin 1a of a rotating friction stir welding tool 1 between the opposing faces of a pair of workpieces 2, 4 while urging the workpieces together. Friction stir welding can also be used to repair cracks or other defects in a single workpiece. The rotation of the threaded pin 1a between the opposing faces of the workpieces 2, 4, or within a single workpiece, creates friction that generates sufficient heat energy to plasticize the workpiece material in the weld zone 6. The friction stir welding tool 1 also includes a shoulder that can have a variety of configurations adapted to consolidate the plasticized workpiece material within the weld zone 6 as the friction stir welding tool is moved along the interface 3 between workpieces or through a single workpiece. A friction stir weld joint 8 forms, joining the workpieces together in a unitary assembly, as the plasticized regions of the workpieces 2, 4 flow together and cool in the weld zone 6. See U.S. Pat. No. 5,460,317 to Thomas et al. for a general discussion of friction stir welding, the entire contents of which are incorporated herein by reference.
One particular benefit of friction stir welding is that the formation of the weld joint 8 is autogenous and is created by the cooling of the plasticized parent materials rather than a filler material, as is commonly used in conventional welding processes. In addition, the friction stir weld joint 8 comprises a nugget having a refined grain structure with grains having an equiaxed shape and grain sizes ranging in order of magnitude from approximately 0.0001 to 0.0002 inches (approximately 3 to 5 microns). As a result of the improved grain structure, the friction stir weld joint 8 resists the formation and propagation of micro-cracks and exhibits improved strength, ductility and toughness, as well as improved corrosion and fatigue resistance.
Unfortunately, several significant problems have limited the application of welding for certain manufacturing processes. One problem generally associated with welding is that the temperature required to melt or plasticize the parent materials typically degrades the material properties of the materials. For example, as shown in FIG. 1, during friction stir welding, the frictional heat created by the rotating friction stir welding tool 1 is conducted from the weld zone 6 through the workpieces 2, 4 into the ambient environment, creating a heat-affected region 7 around the weld zone 6. The elevated temperatures associated with the friction stir welding process can degrade the material properties of the parent materials, including the strength, stiffness, and ductility of the workpieces 2, 4. In addition, the thermal transient created in the workpieces during friction stir welding can result in the weld zone 6 becoming more sensitive to corrosive attack.
Another common problem associated with welding is the formation of tensile residual stresses in the workpieces during the welding process. The tensile residual stresses form as a result of the expansion and then contraction of the regions of the workpieces or workpiece adjacent to the weld joint. Such tensile residual stresses are known to reduce both fatigue life and increase sensitivity to corrosion-fatigue and stress-corrosion cracking in a wide variety of materials. It also has been found that micro-segregation kinetics found in some aluminum alloys, which are commonly used in the aircraft industry, are sufficiently rapid such that stress-corrosion resistance is reduced even after a short thermal transient. Further, where two different workpieces that have different sizes or that comprise different materials are welded together, any residual stress can be amplified due to the difference in heat capacity between the two workpieces.
Another problem associated with many fusion and solid state welding processes is the production of “flash” or excess material at the edge of the weld joint. Fatigue crack initiation can occur out of this area due to the mechanical discontinuity at the edge or “toe” of the weld. This edge or “toe” has been found, in virtually all types of welds, to be the area where the highest tensile residual stresses are found.
In seeking to minimize the degradation of the material properties of fusion and solid state weld joints, several alternative approaches have been proposed. For example, while acceptable corrosion resistance can be achieved by a post-weld heat treatment, e.g., a solution treatment at a predetermined temperature schedule with aging at a second predetermined temperature schedule, post-weld heat treatments are not economically and technically practical except for all but the smallest and simplest of geometric shapes. Further, local heating can result in distortion and increase tensile residual stresses elsewhere in the workpiece.
Other proposed techniques for improving corrosion resistance of a weld joint have included applying a coating, such as paint, electroplating or galvanizing, to all susceptible surfaces of the resulting assembly, including the weld joint. However, such coatings require a second manufacturing operation, which can increase the labor and material cost and production time of the finished assembly. Further, such coatings provide only a superficial protective layer and do not protect surfaces of the assembly that cannot be accessed. In addition, protection of the assembly surface is lost if the coating is broken or deteriorates during service.
Methods of inducing compressive stresses along the surfaces of a workpiece have been used to improve the fatigue life and corrosion resistance of workpiece surfaces. However, such methods for inducing compressive stress in a prescribed pattern along a weld joint or induced along the surface of the resulting welded assembly have not been used or contemplated as a facet of the welding process, since the process for inducing a compressive stress could result in damage or weakening of the weld joint. One such method for inducing a layer of compressive stress in the surface of a workpiece to improve its fatigue life and corrosion resistance is burnishing. The accepted practice for burnishing utilizes repeated deformation of the surface of the part, in order to deliberately cold work the surface of the material to increase the yield strength. Compressive stresses are developed by yielding the surface of the material in tension so that it returns in a state of compression following deformation. However, excess cold working can produce tensile surface residual stresses or spalling damage and can leave the surface susceptible to overload and thermal relaxation.
Other methods commonly used to induce compressive stress in the surface of a workpiece include shot peening, whereby a plurality of metallic or ceramic pellets are projected mechanically or through air pressure to impinge the surface of a workpiece, and gravity peening, whereby pellets are dropped through a chute from a predetermined distance onto the surface of the workpiece. While shot peening and gravity peening may be used for inducing compressive residual stresses along the surface of the weld joint, unfortunately, shot peening and gravity peening also impart an uncontrolled amount of cold work making it difficult to optimize the material properties of the weld joint. Further, the degree of cold working of the material by shot peening or gravity peening is relatively high, which may be undesirable for many applications. In addition, the shot or gravity peening induced compressive residual stresses are relatively shallow, affording little benefit in arresting fatigue or stress corrosion cracks since the shallow compressive layer may be lost to wear or corrosion in service thereby providing little beneficial effects. Shot peening and gravity peening also produce a poor surface finish making the processes unacceptable for many applications. It is also known that the beneficial effects produced by shot peening or gravity peening are generally lost as the pattern of compression relaxes with time, particularly when subjected to elevated temperatures during service.
In addition to material property degradation and the formation of tensile residual stresses, another problem associated with assemblies formed using both solid state and fusion welding techniques is the creation of relatively rough surface finishes. For example, the rubbing and rotation of the shoulder of the friction stir welding tool along the surface of the workpiece typically creates a relatively rough surface finish having a periodic circular pattern. Surface roughness is generally unacceptable for structures used in fatigue loading applications. Accordingly, in order to reduce surface roughness, the travel speed of the rotating friction stir welding tool is often decreased. However, decreasing the travel speed of the tool can significantly increase welding time. Surface roughness of a workpiece also can be reduced by post-weld machining. However, post-weld machining can thin the weld joint and requires a relatively labor-intensive and time-consuming secondary manufacturing operation, which will increase the cost and production time of the finished assembly. In addition, due to the differences in material hardness proximate to the weld joint that result from the soft heat affected zone, it can be difficult to prevent local undercutting when performing the post-weld machining, which can result in fatigue initiation sites.
It should now be apparent that until now, in addition to the problems identified above, all post welding procedures have required a second-pass operation that typically increases the labor and/or material cost and production time of the finished assembly. Consequently, there remains a need for improved methods and apparatus for reducing material property degradation, tensile residual stresses, and surface roughness when forming a weld joint. Such manufacturing methods and apparatus should provide reduced labor costs and production times over conventional methods and should provide a weld joint having improved mechanical and chemical properties, including corrosion resistance, yield strength, hardness, ductility, fatigue life, and surface finish.